Add the decision-complete MVP product and architecture documentation

[codeacme17]

Tracks #1.

Summary

Add the decision-complete MVP product specification, canonical domain language, technical architecture, and accepted architecture decisions to the repository before implementation begins.

Scope

• Add CONTEXT.md as the implementation-independent glossary for Repository, Worktree, provenance, Git state, operations, and outcomes.
• Add docs/product/mvp-prd.md with the complete scope, functional requirements, safety contract, capability matrix, non-functional requirements, and 24 release-blocking acceptance scenarios.
• Add docs/architecture/mvp-technical-architecture.md with the process model, deep module interfaces, protocol, Git execution contract, refresh/concurrency model, security boundary, testing strategy, and implementation phases.
• Add ADRs for isolating Codex host integration and using the system Git CLI behind a local Repository Engine.
• Link the documents from the repository README.

Acceptance criteria

• The checked-in documents preserve all decisions captured in Tracking: Deliver the Codex Git macOS MVP #1 without introducing new product behavior.
• CONTEXT.md contains domain language only and no implementation details.
• Architecture documents distinguish public/official Codex capabilities from the unsupported CDP/DOM adapter.
• Relative links between README, product documentation, architecture, and ADRs resolve correctly on GitHub.
• GitHub-facing content and code examples are in English and contain no generated-by or co-author notices.

Out of scope

• Runtime scaffolding or executable product code.
• Reopening resolved MVP product decisions.

Verification

• Render all Markdown on GitHub and verify headings, tables, diagrams, and links.
• Cross-check the 24 acceptance scenarios against Tracking: Deliver the Codex Git macOS MVP #1.